Even though the hydrolysis of formals and other acetals is one of the fundamental reactions of organic chemistry, problems can still occur where the equilibrium position is one-sided, as is the case, for example, with cyclic acetals having a 1,3-dioxo structure, owing to inadequate selectivity and side reactions, including thermal decomposition.
Linear and cyclic acetals, especially formals, are formed as undesired secondary products in the large-scale preparation of polyhydric alcohols, such as pentaerythritol, trimethylol-propane, trimethylol-ethane, neopentyl glycol and oligomers of those alcohols, which includes an aldol addition and a Cannizzaro reaction. In the Cannizzaro reaction, which is carried out in the presence of an alkali metal, alkaline earth metal or ammonium hydroxide, the corresponding formate is formed. The formate is present in the various stages of working up of the polyols in varying amounts.
WO 97/01523 describes a process for the cleavage, by hydrogenation, of cyclic formals, such as are formed as secondary products in the case of an aldol addition with a subsequent Cannizzaro reaction to prepare polyhydric alcohols, such as pentaerythritol and trimethylolpropane, to the underlying diols and methanol. The cleavage takes place in the aqueous phase at a pH of from 1 to 6 in the presence of a heterogeneous metal catalyst at from 100.degree. to 200.degree. C. under a hydrogen atmosphere. As is shown by Example 1 of that document, a previously acidified aqueous reaction mixture from the preparation of di-trimethylolpropane (di-TMP), which also contains sodium formate in addition to the cyclic formals di-TMP and tri-TMP to be cleaved and unidentified compounds, is hydrogenolyzed in the presence of Ru on activated carbon at 67 bar H.sub.2 and 130.degree. C.
As was discovered by the inventors of the present Application when re-working WO 97/01523, the hydrogenolysis under the mentioned pressure and temperature conditions can only be carried out successfully if it is based on a very high weight ratio of catalytically active metal to cyclic formal. This ratio is 0.91 in the Example mentioned. The formate contained in the reaction mixture evidently acts as a catalyst poison. However, such a high relative amount of catalyst used renders the process uneconomical. Therefore, the object of the invention is to improve the above-mentioned process.